As a technology for realizing high frequency-utilization efficiency and high-speed transmission so as to address the tightening of frequency resources as a result of the increase in the amount of data communicated in wireless communication systems, such as cellular systems, researches on MIMO (Multiple-Input Multiple-Output) transmission are being actively conducted. According to MIMO, multiple transmission signals (transmission streams) are spatially multiplexed by using a plurality of transmission antennas of a transmission apparatus. As a type of MIMO transmission, a technology called “network MIMO” or “CoMP (Coordinated Multi Point Transmission)” by which multiple transmission signals are spatially multiplexed and transmitted from a plurality of transmission apparatuses is also gaining attention.
Among such MIMO transmission technologies, single user-MIMO (SU-MIMO), by which multiple transmission signals addressed to a single reception apparatus having a plurality of reception antennas are spatially multiplexed and transmitted simultaneously, enables a significant increase in the transmission rate of each transmission apparatus. Thus, this technology can be very effectively employed when high transmission rates are required, such as for transmission of moving images. Meanwhile, downlink multi user-MIMO (MU-MIMO) is a technology by which transmission signals addressed to a plurality of reception apparatuses are spatially multiplexed and transmitted simultaneously. This technology enables transmission that effectively utilizes the transmission sources, such as the transmission antennas on the transmitting side or the transmission apparatuses of adjacent cells, even when the number of reception antennas with which each reception apparatus is provided is small. In this technology, a multi-user diversity effect can be obtained by appropriately selecting the reception apparatuses for spatial multiplexing. Thus, MU-MIMO is gaining attention as a technology for increasing frequency utilization efficiency. Uplink MU-MIMO is a technology by which different signals are simultaneously transmitted from a plurality of transmission apparatuses to a reception apparatus provided with a plurality of reception antennas. This technology, which effectively utilizes the reception antennas of the reception apparatus, can also increase frequency utilization efficiency as in the case of downlink MU-MIMO.
Because such MIMO transmissions involve the transmission of a plurality of transmission signals by the same resource, precoding may be performed in advance on the transmitting side so as to prevent interference of reception signals on the receiving side. Normally, precoding is performed on the basis of the propagation path condition on the receiving side. Thus, in a FDD (Frequency Division Duplex) system, the propagation path is measured on the receiving side, and the result of measurement is fed back to the transmitting side as CSI (Channel State Information).
As a method for feeding back the CSI, the following two types have been proposed. One is a method by which the result of measurement of the propagation path is fed back as CSI (which may be referred to as “explicit feedback”). In this method, the result of measurement of an instantaneous propagation path or the result of measurement of an average propagation path over a long time is quantized and fed back. Because the propagation path measured by the reception apparatus is represented by amplitude and phase (vector), one (point A) that is the closest to the result of measurement (point indicated by a white dot) is selected from among candidates depicted in FIG. 1 (points indicated by black dots) and fed back to the transmitting side as the CSI (see Non-patent Document 1).
In the example of FIG. 1, 16 vectors are set in advance as propagation path candidates, so that four bits are required for selecting and feeding back any one point. When, for example, the number of the transmission antennas with which the transmission apparatus is equipped is four, and the number of the reception antennas with which the reception apparatus is equipped is two, a total of eight propagation paths×four bits=32 bits are required for feeding back all of the 4×2=8 propagation paths. In this reception apparatus, when a method by which reception signals are composed by multiplying signals received by the two reception antennas by a reception weight is used, an appropriate point among the candidate points depicted in FIG. 1 may be selected and fed back on the basis of an equivalent propagation path after multiplying by the reception weight. In this case, four propagation paths×four bits=16 bits are required for feedback.
As a feedback method different from the explicit feedback, a method (which may be referred to as “implicit feedback”) is known by which candidates of vectors used for precoding on the transmitting side are determined in advance on the transmitting and receiving sides as known, and information about a vector that can be received with the most favorable characteristics among the candidates is fed back. For example, when the candidates of vectors (which may be referred to as a codebook) used for precoding are represented by the following 16 vectors, the vector with which the most favorable reception characteristics can be obtained is selected in each reception apparatus and fed back (see Non-patent Document 2 below).
                                          (                                                            1                                                                              1                                                                              1                                                                              1                                                      )                    ⁢                      (                                                            1                                                                              j                                                                                                  -                    1                                                                                                                    -                    j                                                                        )                    ⁢                      (                                                            1                                                                                                  -                    1                                                                                                1                                                                                                  -                    1                                                                        )                    ⁢                      (                                                            1                                                                                                  -                    j                                                                                                                    -                    1                                                                                                j                                                      )                    ⁢                      (                                                            1                                                                                                                        1                      +                      j                                                              2                                                                                                                    j                                                                                                                                                -                        1                                            +                      j                                                              2                                                                                            )                    ⁢                      (                                                            1                                                                                                                                                -                        1                                            +                      j                                                              2                                                                                                                                        -                    j                                                                                                                                          1                      +                      j                                                              2                                                                                            )                    ⁢                      (                                                            1                                                                                                                                                -                        1                                            -                      j                                                              2                                                                                                                    j                                                                                                                        1                      -                      j                                                              2                                                                                            )                    ⁢                      (                                                            1                                                                                                                        1                      -                      j                                                              2                                                                                                                                        -                    j                                                                                                                                                                  -                        1                                            -                      j                                                              2                                                                                            )                    ⁢                      (                                                            1                                                                              1                                                                                                  -                    1                                                                                                                    -                    1                                                                        )                    ⁢                      (                                                            1                                                                              j                                                                              1                                                                              j                                                      )                    ⁢                      (                                                            1                                                                                                  -                    1                                                                                                                    -                    1                                                                                                1                                                      )                    ⁢                      (                                                            1                                                                                                  -                    j                                                                                                1                                                                                                  -                    j                                                                        )                          ⁢                                  ⁢                              (                                                            1                                                                              1                                                                              1                                                                                                  -                    1                                                                        )                    ⁢                      (                                                            1                                                                              1                                                                                                  -                    1                                                                                                1                                                      )                    ⁢                      (                                                            1                                                                                                  -                    1                                                                                                1                                                                              1                                                      )                    ⁢                      (                                                            1                                                                                                  -                    1                                                                                                                    -                    1                                                                                                                    -                    1                                                                        )                                              (        1        )            
When the vector is pi (i=1, 2, . . . , 16), and the propagation path matrix measured by the reception apparatus is H, in the reception apparatus in which the signals received by the two reception antennas are combined by MRC (Maximum Ratio Combining), the vector to be fed back is selected according to the following expression (2).
                              argmax                      p            i                          ⁢                                        Hp            i                                                        (        2        )            
The expression (2) indicates that, when a signal precoded with pi on the transmitting side is received via the propagation path H on the receiving side, the vector that maximizes the SNR (Signal to Noise power Ratio) is selected. It should be noted, however, that in a reception apparatus that combines the reception signals by a process such as MMSE (Minimum Mean Square Error), an appropriate precoding vector needs to be selected according to a reference corresponding to the reception process used, instead of the expression (2). Further, when two streams of transmission and reception are performed, the receiving side needs to select two vectors. When two vectors are selected from 16 candidates and information representing the vectors (such as an index for each vector) is fed back to the transmitting side, as in the present example, 2×4 bits=8 bits are required; when one vector is selected and information representing the vector is fed back to the transmitting side, 1×4 bits=4 bits are required.
Thus, when the vector that directly represents the measured propagation path is fed back (explicit feedback), the amount of feedback generally tends to be increased compared with the case where a vector with which favorable characteristics can be obtained is selected from among predetermined precoding vectors and fed back (implicit feedback). However, the actual propagation path cannot be known on the transmitting side in the case of implicit feedback. In contrast, it is known that when explicit feedback enables the transmitting side to know the propagation path upon actual reception of a signal on the receiving side, so that precoding can be performed accordingly and more favorable characteristics can be obtained. Thus, it can be said that explicit feedback and implicit feedback have a trade-off relationship from the viewpoints of transmission characteristics and the amount of feedback.